A wide variety of compositions have been developed for use in seats, cushions mattresses, fitting pads, athletic equipment (including impact absorbing materials), prosthetic devices and similar apparatus which are placed in contact with the human body. Such compositions provide form support, comfort and protection because they have the ability to deform in response to continuously applied pressure or the ability to absorb significant amounts of energy from rapidly applied pressure (impact). In addition, it is a desirable feature for these compositions to be lightweight. This can be achieved to some degree by employing foams or composite materials.
A review of the prior art reveals many examples of materials for use in cushioning and padding applications. These materials generally fall into one or two categories: conventional foams and viscous liquids.
Foams offer the advantages of low cost, lightweight and the ability to exhibit a wide variety of physical properties such as: flexible to rigid, instantaneous or delayed recovery and closed or open cell (breath ability). On the other hand, foams do not flow and therefore are not pressure compensating. In addition, foams do not dissipate energy in impact situations.
Viscous liquids may be either water based or oil based. Generally water based systems are produced by dissolving a water-soluble polymer to increase the viscosity to produce a thick flowable liquid or a gel. While these systems offer pressure compensation in applications such as conforming cushions, they have a specific gravity of about 1.0 versus foams which can be produced with specific gravity of 0.2 or less. Water based systems, when used in cushioning applications, must be protected against evaporation, freezing and microbial growth. Use of an oil-based system overcomes the deficiencies noted for water-based systems. Examples of oil-based materials would include silicon oils, hydrocarbon oils, mineral oil and synthetic polymers such a polyamides and polyglycols. The useful viscosity range for these oils range from about 1,000 to up to 1,000,000 centipoise, depending on the application and the components that are utilized in formulation.
U.S. patents to Terrence M. Drew et al. issued Mar. 3, 1992 (U.S. Pat. No. 5,093,138) and Mar. 31, 1992 (U.S. Pat. No. 5,100,712) describe a flowable, pressure compensating composition including water, a material for increasing the viscosity of water, and spherical particles dispersed through-out the volume of the water. The composition disclosed in these patents is a deformable fluid that has the disadvantages of substantial weight, memory, and being slow to flow or shear in response to a deforming pressure.
U.S. patents to Chris A. Hanson issued Oct. 22, 1991 (U.S. Pat. No. 5,058,291) and Aug. 28, 1990 (U.S. Pat. No. 4,952,439) describe padding devices, which are resistant to flow in response to an instantly applied pressure. The composition of the padding material is a combination of wax and discrete particles, including microspheres. The padding disclosed in these patents has the disadvantage of being slow to flow in response to pressure, thus having a high shearing force. The materials disclosed in these patents also have memory, causing them to tend to return to their original shape after removal of a deforming pressure. Memory is described in U.S. patents to Chris A. Hanson issued Sep. 15, 1992, (U.S. Pat. No. 5,147,685), Terrence M. Drew issued Apr. 20, 1993 (U.S. Pat. No. 5,204,154), Chris A. Hanson issued Aug. 28, 1990 (U.S. Pat. No. 4,952,439), Thomas F. Canfield issued Sep. 22, 1970 (U.S. Pat. No. 3,529,368), Terrence M. Drew et al. issued Mar. 3, 1992 (U.S. Pat. No. 5,093,138), Chris A. Hanson issued Oct. 22, 1991 (U.S. Pat. No 5,058,291) and Terrence M. Drew, et al. issued Mar. 31, 1992 (U.S. Pat. No. 5,100,712).
U.S. patents to Eric C. Jay issued Mar. 1, 1998 (U.S. Pat. No. 4,728,551), Jack C. Swan, Jr. issued Oct. 21, 1980 (U.S. Pat. No. 4,229,546), Jack C. Swan, Jr. issued Aug. 2, 1977 (U.S. Pat. No. 4,038,762), Henry Winfred Lynch issued Oct. 19, 1976 (U.S. Pat. No. 3,986,213) and Frederick L. Warner issued Jul. 31, 1973 (U.S. Pat. No. 3,748,669), disclose pressure-compensating mixtures, which are generally characterized by having a quantity of micro beads dispersed in a flowable liquid medium.
Disadvantages of such mixtures include their weight, head pressure and memory. The liquid described in those patents is formulated for certain flow characteristics and the micro beads are merely added because of their low specific gravity to reduce the total weight of the mixture. The resulting mixture is still very heavy because the light micro beads are not used to replace a substantial amount of the heavy liquid, but are instead used only to provide a slight weight reduction of the mixture compared to the use of a liquid alone.
U.S. patents to Tony M. Pearce issued Jun. 6, 1995 (U.S. Pat. No. 5,421,874), Aug. 27, 1996 (U.S. Pat. No. 5,549,743), May 6, 1997 (U.S. Pat. No. 5,626,657), Feb. 1, 2000 (U.S. Pat. No. 6,020,055), describe a composite mixture of spherical objects and lubricant useful for its cushioning properties. The composite may be composed of microspheres and any of a variety of lubricants that involves sliding and rolling contact of the spherical particles with respect to each other. This creates a situation where interactions between spherical particles are avoided. The result is the inability to transfer localized loading through out the composite material and can lead to xe2x80x98bottoming outxe2x80x99 of the cushion or padding device.
U.S. patent to Lincoln P. Nickerson, issued Nov. 8, 1994 (U.S. Pat. No. 5,362,543), describes a composite composition comprising a silicone fluid with an amide thickener filled with glass or phenolic micro-spheres. Their compositions are particularly characterized by their ability to flow in response to a continuously applied pressure, yet to maintain their shape and position in the absence of applied pressure.
The use of block copolymers in padding and cushioning compositions is described in U.S. patents issued to Terry M. Pearce, issued Feb. 22, 2000 (U.S. Pat. No. 6,026,527), and to Lincoln P. Nickerson issued Feb. 9, 1999 (U.S. Pat. No. 5,869,164). These patents disclose the use of ABA type Block copolymers, generally composed of one block of polystyrene and the other block of a soft rubber like elastomer. The addition of these block copolymers to oil based vehicles results in a thixotropic fluid. Microspheres are utilized to lower the density of these compositions.
U.S. patent to Philip Schaefer, issued Feb. 24, 1981 (U.S. Pat. No. 4,252,910) describes a material for use in resilient conforming pads, cushions and the like. The material comprises plastic micro-spheres cohered to a mass by a thermoplastic bonding agent. The bonding agent is a polybutene polymer in the molecular weight range of 3,000 to 7,500. Given this relatively narrow molecular weight range the viscous-elastic properties of the Schaefer composition are rather limited. In fact Schaefer states that the bonding agent is flowable plastic at about, or slightly higher than body temperature. Given these conditions and restrictions the Schaefer invention is quite limited in its scope.
The present invention is directed to improved, lightweight compositions for padding and cushioning devices. These compositions comprise three components: a paraffinic-based fluid, as a vehicle; micro-spheres, as a lightweight included phase; and a high molecular weight rheological modifier. Furthermore, it is an important aspect of this invention that both the fluid based vehicle and the rheological modifier provide a degree of bonding or adhesion to the microspheres to provide a means of attaining improved response and distribution of mechanical loads. In addition, improved bonding or adhesion also provides for better stability of the composition, that is, better resistance to separation of the microspheres from continuous vehicle phase.
A variety of fluids can be used as the vehicle. The preferred fluids are saturated polyalphaolefins, paraffinic mineral oils and polybutene fluids including polyisobutylene and poly-1-butene.
Microspheres serve to significantly lower the density of the composite material. Preferred microspheres include those formed from phenolic or other plastic materials, glass or ceramic materials. Plastic microspheres are generally preferred because they are considerably lighter than glass or ceramic microspheres. Plastic microspheres offer additional benefits in that they can undergo instantaneous compression and recovery for impact padding uses.
The rheological modifying agent of this invention is polybutene having a molecular weight of about 750,000 to about 5,000,000, preferably about 1,000,000 to about 2,000,000, most preferably about 1,000,000.
The compositions of the present invention are especially useful as filling materials for deformable, pressure compensating padding devices comprising a flexible protective envelope having a cavity which contains the composition and which envelope has structure that allows the composition to deform in the cavity in response to a continuously applied load upon said envelope, but to maintain position in the absence of pressure.
In the case where the applied load is instantaneous, such as in impact, said envelope deforms minimally and the transferred energy is effectively dissipated by the contained composition. The compositions of the present invention are particularly characterized by their:
1. Ability to deform by flowing in response to a continuously applied pressure.
2. Ability to dissipate or absorb the kinetic energy, which results from an impact event.
3. Tendency to maintain shape and position in the absence of an applied pressure.
4. Lack of resiliency, under loading normally associated with materials such foams or elastomers.
5. Minor changes in viscosity when subjected to changes in temperature.
6. Resistance to phase separation of the vehicle and microsphere components.
7. Chemical compatibility with vinyl, polyurethane and polyolefin films.
8. Excellent biocompatibility that is non-poisonous and low probability of contact dermatitis.
9. Low potential for microbial growth.
10. Stable over time, that is, long shelf life.